Andersson et al. 2004. Tropical savannah woodland: effects of experimental fire on soil microorganisms and soil emissions of carbon dioxide

Andersson, M., Michelsen, A., Jensen, M., Kjøller, A., 2004. Tropical savannah woodland: effects of experimental fire on soil microorganisms and soil emissions of carbon dioxide. Soil Biol. Biochem. 36, 849-858.

Abstract

Burning of the vegetation in the African savannahs in the dry season is widespread and may have significant effects on soil chemical and biological properties. A field experiment in a full factorial randomised block design with fire, ash and extra grass biomass as main factors was carried out in savannah woodland of the Gambella region in Ethiopia. The microbial biomass C (C_mic) was 52% (fumigation–extraction) and 20% (substrate-induced respiration) higher in burned than unburned plots 12 d after burning. Both basal respiration and potential denitrification enzyme activity (PDA) immediately responded to burning and increased after treatment. However, in burned plots addition of extra biomass (fuel load) led to a reduction of C_mic and PDA due to enhanced fire temperature. Five days after burning, there was a short-lived burst in the in situ soil respiration following rainfall, with twice as high soil respiration in burned than unburned plots. In contrast, 12 d after burning soil respiration was 21% lower in the burned plots, coinciding with lower soil water content in the same plots. The fire treatment resulted in higher concentrations of dissolved organic C (24–85%) and nitrate (47–76%) in the soil until 90 d after burning, while soil NH₄⁺–N was not affected to the same extent. The increase in soil NO₃⁻–N but not NH₄⁺–N in the burned plots together with the well-aerated soil conditions indicated that nitrifying bacteria were stimulated by fire and immediately oxidised NH₄⁺–N to NO₃⁻–N. In the subsequent rainy season, NO₃⁻–N and, consequently, PDA were reduced by ash deposition. Further, C_mic was lower in burned plots at that time. However, the fire-induced changes in microbial biomass and activity were relatively small compared to the substantial seasonal variation, suggesting transient effects of the low severity experimental fire on soil microbial functioning.

Gerschlauer et al. 2014. Greenhouse gas exchange in tropical mountain ecosystems in Tanzania

Gerschlauer et al. 2014. Greenhouse gas exchange in tropical mountain ecosystems in Tanzania. EGU General Assembly 2014, held 27 April - 2 May, 2014 in Vienna, Austria

Abstract
Tropical mountain ecosystems with their mostly immense biodiversity are important regions for natural resources but also for agricultural production. Their supportive ecosystem processes are particularly vulnerable to the combined impacts of global warming and the conversion of natural to human-modified landscapes. Data of impacts of climate and land use change on soil-atmosphere interactions due to GHG (CO2, CH4, and N2O) exchange from these ecosystems are still scarce, in particular for Africa. Tropical forest soils are underestimated as sinks for atmospheric CH4 with regard to worldwide GHG budgets (Werner et al. 2007, J GEOPHYS RES Vol. 112). Even though these soils are an important source for the atmospheric N2O budget, N2O emissions from tropical forest ecosystems are still poorly characterized (Castaldi et al. 2013, Biogeosciences 10). To obtain an insight of GHG balances of selected ecosystems soil-atmosphere exchange of N2O, CH4 and CO2 was investigated along the southern slope of Mt. Kilimanjaro, Tanzania. We will present results for tropical forests in three different altitudes (lower montane, Ocotea, and Podocarpus forest), home garden (extensive agro-forestry), and coffee plantation (intensive agro-forestry). Therefore we used a combined approach consisting of a laboratory parameterization experiment (3 temperature and 2 moisture levels) and in situ static chamber measurements for GHG exchange. Field measurements were conducted during different hygric seasons throughout two years. Seasonal variation of temperature and especially of soil moisture across the different ecosystems resulted in distinct differences in GHG exchange. In addition environmental parameters like soil bulk density and substrate availability varying in space strongly influenced the GHG fluxes within sites. The results from parameterization experiments and in situ measurements show that natural forest ecosystems and extensive land use had higher uptakes of CH4. For the investigated forest ecosystems we found considerable differences in soil sink strength for CH4. N2O emissions were highest in natural forest ecosystems even though N input in the intensively managed system was considerably higher. Highest N2O efflux rates were identified in the region of highest mean annual precipitation. CO2 emissions reduced from managed to natural ecosystems. In general an increase in temperature as well as in soil moisture caused higher GHG fluxes throughout all investigated natural and managed ecosystems. With increasing altitude of the investigated forests GHG emissions reduced overall. 

http://meetingorganizer.copernicus.org/EGU2014/EGU2014-10821.pdf

Rabenarivo et al. 2014. Emissions of CO2 and N2O from a pasture soil from Madagascar—Simulating conversion to direct-seeding mulch-based cropping in incubations with organic and inorganic inputs. J. Plant Nutr. Soil Sci.

Rabenarivo, M., Wrage-Moennig, N., Chotte, J.-L., Rabeharisoa, L., Razafimbelo, T.M., Chapuis-Lardy, L., Emissions of CO2 and N2O from a pasture soil from Madagascar—Simulating conversion to direct-seeding mulch-based cropping in incubations with organic and inorganic inputs. J. Plant Nutr. Soil Sci. DOI: 10.1002/jpln.201300032

 

Abstract

In the highlands of Madagascar, agricultural expansion gained on grasslands, and cropping systems based on direct seeding with permanent vegetation cover are emerging as a means to sustain upland crop production. The objective of this study was to examine how such agricultural practices affect greenhouse-gas emissions from a loamy Ferralsol previously used as a pasture. We conducted an experiment under controlled laboratory conditions combining cattle manure, crop residues (rice straw), and mineral fertilizers (urea plus NPK or di-NH₄-phosphate) to mimic on-field inputs and examined soil CO₂ and N₂O emissions during a 28-d incubation at low and high water-filled pore space (40% and 90% WFPS). Emissions of N₂O from the control soil, i.e., soil receiving no input, were extremely small (< 5 ng N₂O-N (g soil)^–1 h^–1) even under anaerobic conditions. Soil moisture did not affect the order of magnitude of CO₂ emissions while N₂O fluxes were up to 46 times larger at high soil WFPS, indicating the potential influence of denitrification under these conditions. Both CO₂ and N₂O emissions were affected by treatments, incubation time, and their interactions. Crop-residue application resulted in larger fluxes of CO₂ but reduced N₂O emissions probably due to N immobilization. The use of di-NH₄-phosphate was a better option than NPK to reduce N₂O emissions without increasing CO₂ fluxes when soil received mineral fertilizers. Further studies are needed to translate the findings to field conditions and relate greenhouse-gas budgets to crop production.

Thomas and Hoon, 2010. Carbon dioxide fluxes from biologically-crusted Kalahari Sands after simulated wetting.

Thomas, A.D., Hoon, S.R.,2010. Carbon dioxide fluxes from biologically-crusted Kalahari Sands after simulated wetting. J. Arid Environ. 74, 131-139.

Abstract

We report surface CO₂ efflux and subsoil CO₂ concentrations in biologically-crusted soils from the Kalahari. Fluxes were determined in-situ using a closed chamber coupled to a portable gas chromatograph on dry soils and on soils subject to simulated light and heavy rainfall. Surface efflux was measured in an artificially darkened environment in order to determine by difference, whether photosynthesis was occurring. Dry soil efflux rates were 2.8–14.8 mg C m² h⁻¹ throughout a diurnal cycle. Light rainfall led to an immediate increase in efflux to a peak of 65.6 mg C m² h⁻¹. Heavy rainfall resulted in a large pulse of CO₂ with efflux rates of 339.2 mg C m² h⁻¹ over the first hour after wetting. Peak rates remained high over the following 2 days (87.8 and 87.0 mg C m² h⁻¹). Given sufficient moisture, fluxes increased with temperature. We believe hydration of the subsoil stimulates microorganisms which repsire available C either from extracellular polysaccharide sheaths (EPS) or released into the soil through lysis of microbial cells. Higher fluxes from the soil kept in the dark suggests photosynthesis occurs in wetted crusts during the daytime but net C uptake is masked by respiration from other microorganisms.

Thomas et al. 2008. Carbon dioxide fluxes from cyanobacteria crusted soils in the Kalahari.

Thomas, A.D., Hoon, S.R., Linton, P.E., 2008. Carbon dioxide fluxes from cyanobacteria crusted soils in the Kalahari. Applied Soil Ecology 39, 254-263.

Abstract

The surface of dryland soils is frequently characterised by a biological crust comprising of various combinations of cyanobacteria, algae, moss and lichens. In the Kalahari of Botswana, soil crusts are predominantly made up of cyanobacteria, which when moist, are capable of fixing N and C. Many cyanobacteria also produce extracellular polymeric substances (EPS) which bind soil particles together and decrease erodibility. The physical integrity and metabolic activity of soil crusts is thus critical to ecological productivity, erodibility and CO₂ fluxes in dryland regions. There are, however, few studies of the magnitude and controlling factors of soil CO₂ flux within these systems.

Our aim was to quantify in situ soil CO₂ flux during contrasting antecedent moisture conditions in the south west Kalahari of Botswana. We have designed a gas exchange chamber for field deployment coupled to a portable gas chromatograph, control and data logging instrumentation. The optical and active thermal control specifications of the chamber have been designed to permit photosynthesis and cope with the temperature extremes of the Kalahari whilst minimizing disturbance to the cyanobacteria soil crust. This approach has enabled CO₂ fluxes to be monitored in situ with a high degree of precision for extended periods.

In August 2005, when the surface and subsoils were dry, the ambient CO₂ efflux was negative and low during the daytime (−6.15 mg C m² h⁻¹). When 8 mm rainfall equivalent of water was added to the surface there was an immediate uptake of CO₂ during the daytime at rates up to 75 mg C m² h⁻¹ demonstrating that rates of net photosynthesis are greatly enhanced by available moisture. In contrast, in May 2006 following a prolonged wet period when the subsoil was moist, there was a net positive efflux of CO₂ from the soil at rates of up to 60 mg C m² h⁻¹ irrespective of whether the surface soil was moist or not. This is consistent with subsoil heterotrophic bacterial respiration becoming an important contributor to soil efflux.

Thomas et al., 2012. Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana.

 Thomas, A.D., 2012. Impact of grazing intensity on seasonal variations in soil organic carbon and soil CO2 efflux in two semiarid grasslands in southern Botswana. Philosophical Transactions of the Royal Society B: Biological Sciences 367, 3076-3086.

Abstract

Biological soil crusts (BSCs) are an important source of organic carbon, and affect a range of ecosystem functions in arid and semiarid environments. Yet the impact of grazing disturbance on crust properties and soil CO₂ efflux remain poorly studied, particularly in African ecosystems. The effects of burial under wind-blown sand, disaggregation and removal of BSCs on seasonal variations in soil CO₂ efflux, soil organic carbon, chlorophyll a and scytonemin were investigated at two sites in the Kalahari of southern Botswana. Field experiments were employed to isolate CO₂ efflux originating from BSCs in order to estimate the C exchange within the crust. Organic carbon was not evenly distributed through the soil profile but concentrated in the BSC. Soil CO₂ efflux was higher in Kalahari Sand than in calcrete soils, but rates varied significantly with seasonal changes in moisture and temperature. BSCs at both sites were a small net sink of C to the soil. Soil CO₂ efflux was significantly higher in sand soils where the BSC was removed, and on calcrete where the BSC was buried under sand. The BSC removal and burial under sand also significantly reduced chlorophyll a, organic carbon and scytonemin. Disaggregation of the soil crust, however, led to increases in chlorophyll a and organic carbon. The data confirm the importance of BSCs for C cycling in drylands and indicate intensive grazing, which destroys BSCs through trampling and burial, will adversely affect C sequestration and storage. Managed grazing, where soil surfaces are only lightly disturbed, would help maintain a positive carbon balance in African drylands.

Thomas et al. 2011. Soil respiration at five sites along the Kalahari Transect: Effects of temperature, precipitation pulses and biological soil crust cover.

Thomas, A.D., Hoon, S.R., Dougill, A.J., 2011. Soil respiration at five sites along the Kalahari Transect: Effects of temperature, precipitation pulses and biological soil crust cover. Geoderma 167-168, . 284-294.

Abstract

There are increasing concerns that climatic and land use changes will enhance soil respiration rates and soil organic carbon loss, compromising agricultural productivity and further elevating atmospheric CO₂. Current understanding of dryland respiration is, however, insufficient to enable prediction of the consequences of these changes for dryland soils and CO₂ fluxes. The objectives of this paper are to present in-situ respiration data from five remote sites along a climatic gradient in the Kalahari of Botswana and to determine the effects of temperature, moisture and biological crust cover on soil CO₂ fluxes. Moisture was the primary limiting factor to efflux which increased with amount of simulated rainfall. On dry soils, mean CO₂ efflux was between 1.5 and 5.9 mg C m^− 2 h^− 1. After 2 mm and 50 mm simulated wetting, mean rates increased to 4.0 to 21.8 and 8.6 to 41.5 mg C m^− 2 h^− 1 respectively. Once wet, soil CO₂ efflux increases with temperature, and sites at the hotter northern end of the transect lost more CO₂ than cooler southerly sites. Net respiration rates are, however, muted by autotrophic organisms in biological soil crusts which photosynthesise and take up CO₂. The temperature sensitivity of soil CO₂ efflux increased with moisture. Dry, 2 mm and 50 mm treated soils had a Q₁₀ of 1.1, 1.5 and 1.95 respectively. Our findings indicate that higher temperatures and a loss of biological crust cover will lead to greater soil C loss through respiration.